1. Field of the Invention
This invention relates to the separation of particles from a stream of gas and entrained particles. As used herein, the term "particles concentration" signifies a measure of the quantity of particulate material to the quantity of gas in the stream of a part thereof on a weight or volume basis and not of the actual number of particles in the stream. Thus, a stream which has a high particle concentration is rich in particulate material and lean in gas whereas a stream of low particle concentration is relatively rich in gas and lean in particulate material.
2. Description of Prior Art
One application of the invention is to the burner pipes of brown coal burning boilers. In such boilers, the brown coal is pulverised in a mill and is carried by a stream of hot gas to the burners, the mill providing the active mixing needed for drying. The stream of gas and pulverised fuel is usually passed through a classifier before being supplied to the burner so that oversize fuel is returned to the mill for further treatment. With the high moisture content of the brown coal, large quantities of water vapour are generated which tend to quench the flame. This problem can be overcome by causing a partial segregation of the pulverised fuel from the accompanying gas and water vapour to form separate sub-streams of differing particle concentration, these sub-streams being supplied to separate parts of the furnace to improve the combustion and heat transfer therein.
Conventionally the required partial segregation for separation firing is obtained in a tubular concentrator fitted with radial vanes which superimpose on the flow a swirling motion. Such a concentrator creates large pressure losses in the system and substantial extra power is required to overcome these losses. Moreover the turbulence associated with the swirl tends to oppose the inertial separation of the coal particles.
By the present invention it is possible to create three-dimensional swirling flow which is also a substantially streamline or irrotational flow. Such a flow can provide efficient separation of particles without turbulent re-mixing and with much lower power losses than with conventional cyclone separators.
There have been previous attempts to achieve three-dimensional swirling flow without the turbulence and diffusion associated with conventional cyclone separators. U.S. Pat. No. 2,385,745 of Vogt discloses one construction which attempts to achieve such flow. In this construction there is an annular chamber and a cover which starts at the top of the inlet and spirals downwardly through 360.degree. and it is stated that the resultant downward deflection of gases causes "contiguous convolutions of uniform cross-section without either radial or axial displacement by incoming air." However, the mere provision of a guide surface along a particular desired flow path does not ensure that a real fluid will follow that path. In fact a real flow will only be fully guided by imposed boundaries if the pressure gradients implied by the shape of the guiding boundaries can be sustained in real fluids. Therefore to achieve a substantially streamline flow the shapes of the surfaces confining the flow must be carefully designed to conform with a real flow without unsustainable pressure gradients and all of the surfaces must be compatible with the desired real flow. Specifically, in order to transform a uniform duct flow into a swirling non-turbulent flow it is necessary to provide a carefully designed inlet which will provide a transitional flow consistent with the duct flow and the swirling flow. Leading a uniform duct flow directly into an annular flow as shown in FIG. 2 of the Vogt specification will either result in acceleration of the fluid approaching the centrebody (3) surface -- which is then inconsistent with the flat radial profile of the helical cover -- or, to be consistent, in a vortical, and therefore turbulent flow with a constant swirl component. The practical result will probably be a combination of these two comprising a near inviscid region and an outer vortical region stemming from the flow separating from that wall. The three-dimensional interaction with the incompletely compatible roof would produce further complications resulting in turbulence.
U.S. Pat. No. 2,378,600 (Van Tongeren) and U.S. Pat. No. 3,060,664 (Morawski) both show cyclone separators in which there is provided means intended to impart a helical pattern of rotation in the vicinity of the inlet in such a way as to reduce interference and turbulence at the inlet scroll. However, in these constructions, too, there is no analysis of the flow to be achieved and no attempt to provide an inlet which can provide the necessary transition between a straight duct flow and a swirling flow while maintaining a realisable flow of a real fluid and without unsustainable pressure gradients.
The present invention involves matching of a carefully shaped inlet with an annular duct to conform with a realisable flow involving a transition from a straight duct flow to a swirling irrotational annular flow. Analysis of the flow concerned will be detailed below and it will be seen that it results in an inlet of unique shape which is quite distinct from the inlets of the prior art constructions.